Make tab and tabset context menus extensible

Update code for creating context menus to use *-menu-items triggers.
Add :order key so item order can be specified by other plugins or
behaviors.
Update rem-tabset def to remove compiler warning
Fix this variable reference in tab-menu-items
Merge remote-tracking branch 'lighttable/master' into extensible-menus

Conflicts:
	deploy/settings/default/default.behaviors
	src/lt/objs/tabs.cljs
Rename tabset menu items trigger
Add object for tab label

Adds an object for the tab label so that the right click menu can be
extended with the :menu! and :menu+ triggers. Also allows use to reuse
the :lt.objs.menu/menu! behavior and reduce code duplication.
Make $ and $$ multi arity

Reduces the number of warnings generated by the cljs compiler.
Merge current master into pull request branch
Merge pull request #1391 from mortalapeman/extensible-menus

Extensible menus